Datasheet LTC1622 (Analog Devices) - 9

LTC1622
U U W U APPLICATIONS INFORMATION
The allowable forward voltage drop in the diode is calcu-  1  lated from the maximum short-circuit current as: ∆V ≈ I ESR OUT RIPPLE +  8 fCOUT  P V D ≈ where f is the operating frequency, C F OUT is the output ISC MAX ( ) capacitance and IRIPPLE is the ripple current in the induc- tor. The output ripple is highest at maximum input voltage where PD is the allowable power dissipation and will be since ∆IL increases with input voltage. determined by efficiency and/or thermal requirements. The choice of using a smaller output capacitance in- A fast switching diode must also be used to optimize creases the output ripple voltage due to the frequency efficiency. Schottky diodes are a good choice for low dependent term, but can be compensated for by using forward drop and fast switching times. Remember to keep capacitors of very low ESR to maintain low ripple voltage. lead length short and observe proper grounding (see The ITH pin OPTI-LOOP compensation components can be Board Layout Checklist) to avoid ringing and increased optimized to provide stable, high performance transient dissipation. response regardless of the output capacitors selected.
CIN and COUT Selection
Manufacturers such as Nichicon, United Chemicon and Sanyo should be considered for high performance through- In continuous mode, the source current of the P-channel hole capacitors. The OS-CON semiconductor dielectric MOSFET is a square wave of duty cycle (VOUT + VD)/ capacitor available from Sanyo has the lowest ESR (size) (VIN + VD). To prevent large voltage transients, a low ESR product of any aluminum electrolytic at a somewhat input capacitor sized for the maximum RMS current must higher price. Once the ESR requirement for COUT has been be used. The maximum RMS capacitor current is given by: met, the RMS current rating generally far exceeds the / IRIPPLE(P-P) requirement. V V [ ( −V )]1 2 OUT IN OUT C Required I I In surface mount applications, multiple capacitors may IN RMS ≈ MAX VIN have to be paralleled to meet the ESR or RMS current This formula has a maximum at VIN = 2VOUT, where IRMS handling requirements of the application. Aluminum elec- = IOUT/2. This simple worst-case condition is commonly trolytic and dry tantalum capacitors are both available in used for design because even significant deviations do not surface mount configurations. In the case of tantalum, it is offer much relief. Note that capacitor manufacturer’s critical that the capacitors are surge tested for use in ripple current ratings are often based on 2000 hours of life. switching power supplies. An excellent choice is the AVX This makes it advisable to further derate the capacitor, or TPS, AVX TPSV and KEMET T510 series of surface mount to choose a capacitor rated at a higher temperature than tantalum, available in case heights ranging from 2mm to required. Several capacitors may be paralleled to meet the 4mm. Other capacitor types include Sanyo OS-CON, Sanyo size or height requirements in the design. Due to the high POSCAP, Nichicon PL series and the Panasonic SP series. operating frequency of the LTC1622, ceramic capacitors can also be used for C
Low Supply Operation
IN. Always consult the manufacturer if there is any question. Although the LTC1622 can function down to 2V, the The selection of C maximum allowable output current is reduced when V OUT is driven by the required effective IN series resistance (ESR). Typically, once the ESR require- decreases below 3V. Figure 3 shows the amount of change ment is satisfied, the capacitance is adequate for filtering. as the supply is reduced down to 2V. Also shown in The output ripple (∆V Figure 3 is the effect of V OUT) is approximated by: IN on VREF as VIN goes below 2.3V. Remember the maximum voltage on the ITH pin defines 9